Make XML composer template slots if needed

[cjcodeproj]

If there is no composer value in the ID3 metadata, the code should provide the option to create dummy entries so it's easier to edit the XML.

Something like:

id3scan --make-composer-templates 2

Which will output something like this.

<composers>
 <composer><gn>PLACEHOLDER</gn><fn>COMPOISER1</fn></composer>
 <composer><gn>PLACEHOLDER</gn><fn>COMPOSER2</fn></composer>
</composers>

The code should support the optional argument of an integer value between 1 and 5m to indicate the number of slots it should make. Also, the code should not generate entries if there is composer metadata already present in the track.